SMC -
Bolt Action is basing agnostic - you can use circles or squares or whatever. For weapons teams they kind of leave it up to you. (Which I personally think is dumb, but from what I understand Warlord is going to start putting plastic bases with EVERYTHING for Bolt Action, which is why they just released their own Warlord Games branded plastic bases in the last week or two)
I use Renedra bases, personally:
http://www.architectsofwar.com/figurebases.aspx
25mm round - infantry
40mm round - weapons teams (
MG, mortar, etc)
60mm round - heavy weapons (heavy mortars, light artillery, light antitank guns)
Large oval - Heavy antitank guns, Heavy antiaircraft, etc